HUNTSVILLE, Texas -- A 61-year-old Texas inmate was executed Wednesday evening for killing a Houston police officer more than three decades ago.

Robert Jennings received lethal injection for the July 1988 fatal shooting of Officer Elston Howard during a robbery at an adult bookstore that authorities said was part of a crime spree.

As witnesses filed into the death chamber, Jennings asked a chaplain standing next to him if he knew the name of the slain officer. The chaplain didn't appear to respond, and a prison official then told the warden to proceed with the punishment.

"To my friends and family, it was a nice journey," Jennings said in his final statement. "To the family of the police officer, I hope y'all find peace. Be well and be safe and try to enjoy life's moments, because we never get those back."

Outside the prison, more than 100 officers stood vigil. And a motorcycle club that supports police revved their engines, with the roar from the bikes audible in the chamber.

Jennings was pronounced dead at 6:33 p.m., 18 minutes after the drug started. He became the first inmate put to death this year both in the U.S. and in Texas, the nation's busiest capital punishment state.

"Justice has been rendered and my family can finally have the closure we deserve," Michael Agee, Howard's nephew and a current Houston officer, said after watching Jennings die.

Houston Police Chief Art Acevedo, asked about the 30-plus years between the crime and the punishment, said he thought "justice delayed is, to an extent, an injustice continued."

"But when the state takes a life, there has to be a process," Acevedo said. "In this case, the day of reckoning is here. It's a solemn occasion. For us it's a celebration of a life well-lived by Officer Howard. We're a family. That's why we're here."

His attorneys had asked the U.S. Supreme Court to delay his execution, arguing Jennings' trial attorneys failed to ask jurors to fully consider evidence - including details of his remorse for the officer's shooting and possible brain damage - that might have spared him a death sentence.

Jennings had received an execution stay in 2016. But the high court and lower appeals courts rejected his request to delay Wednesday's execution and the Texas Board of Pardons and Paroles turned down Jennings' request to commute his sentence.

A twice convicted robber, Jennings had been on parole for about two months when prosecutors say he entered Mr. Peeper's Bookstore with the intention of robbing the business. Since being paroled, Jennings had gone on a crime spree, committing about 10 robberies, including having already robbed the same adult bookstore 12 days before Howard's slaying.

Officer Howard, 24, was in the middle of arresting the store clerk for operating a pornographic video arcade without a permit when Jennings shot the officer twice in the head.

Howard, who had been wearing a jacket with the words "Houston Police" on it, staggered for a few feet before falling to the ground, where he was shot twice more by Jennings. The clerk later testified the shooting was so quick, Howard never had a chance to unholster his gun.

Jennings was arrested hours later when he went to a Houston hospital after being shot in the hand by his accomplice, who got angry at Jennings for shooting the officer.

Jennings would be the first inmate put to death this year both in the U.S. and in Texas, which is the nation's busiest capital punishment state.

Joe Gamaldi, the president of the Houston Police Officers' Union, said Jennings has spent more time on death row than Howard was alive.

Howard "was an honorable man full of integrity who did his job. He was absolutely one of the best and he was just taken entirely too soon by this animal who murdered him in cold blood," Gamaldi said.

After his arrest, Jennings confessed to killing Howard, telling police in a tape-recorded statement he was remorseful about what happened and would "face whatever punishment (he had) coming."

Edward Mallett, one of Jennings' current appellate attorneys, said the inmate's trial attorneys failed to present sufficient evidence of his remorse as well as his history of brain damage, being abused as a child and drug addiction. He said the trial attorneys also failed to provide an instruction to jurors that would have allowed them to give sufficient weight to these aspects of Jennings' life when they deliberated.

Mallett said a prior appellate attorney also failed to argue these issues in earlier appeals.

"There has not been an adequate presentation of his circumstances including mental illness and mental limitations," Mallett said.

Jennings' trial in 1989 took place just as the Supreme Court issued a ruling that faulted Texas' capital sentencing statute for not allowing jurors to consider evidence supporting a sentence less than death.

The Texas Legislature changed the statute to address the high court's concerns but that took place after Jennings was convicted.

The Texas Attorney General's Office called Jennings' claim he had ineffective lawyers at his trial and during earlier appeals "specious," and said appeals courts have previously rejected allegations his personal history was not adequately investigated and presented at his trial.

"My hope is that on Wednesday (Howard's family gets) the closure that they've been searching for 30 years," Gamaldi said.